Process for preparing alkyldihaloarsine



Patented Apr. 28, 1953 PROCESS FOR PREPARING ALKYLDI- HALOARSINE MorrisS. Kharasch, Chicago, 111., and Sidney Weinhouse, Chester, Pa.,

assignors to the United States of America as represented by theSecretary of War No Drawing. Original application February 21,

1944, Serial No.

523,364, now Patent No.

2,615,043, dated October 21, 1952. Divided and this application May 15,1952, Serial No. 291,053

. The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment to us ofany royalty thereon.

This application is a division of our original application entitledChemical Process, filed in the Patent Oflice February 21, 1944, SerialNo. 523,364, now Patent 2,615,043.

This invention relates to a method for the preparation ofethyldichloroarsine and related organo-mineral-halides.

An object of this invention is to provide a method for securing highyields of the desired product and which is well suited for industrialscale manufacture.

Ethyldichloroarsine was introduced as a chemical warfare agent by theGermans in 1918. The best known methods for preparing this compound havebeen essentially the same as the-German process. They are complicatedand involve the following steps: (1) the conversion of ethyl chlorideinto disodium ethyl arsenate; (2) the reduction of disodium ethylarsenate with sulfur dioxide to form ethyl arsenious oxide; (3) thetreatment of ethyl arsenious oxide with hydrogen chloride to formethyldichloroarsine. It has been confirmed that this process gives anaverage over-all yield of only about 27 to 36% at the most.

It is evident that such prior methods are ill suited for large scaleoperation. For this reason a new and radically different method has beendeveloped.

In the new method of this invention, ethyldichloroarsine is prepared bya reaction of arsenic trichloride with tetraethyl lead under suitableconditions.

Theoretically, the over-all reaction is:

However, it is important to note that the reaction evidently takes placein two stages.

The first stage of the reaction is represented by the equation:

and proceeds spontaneously at room temperature or at temperatures below50 C. At these temperatures even an excess of arsenic trichloride failsto detach a third ethyl radical from the lead atom.

, i The second stage represented by:

proceeds slowly at 80 C. and rapidly at temperatures above 90 C.

6 Claims. (01. 260440) The reactions may be carried out in the presenceof suitable solvents, and the over-all reaction may be carried out inthe absence of solvents. When low-boiling solvents are used, only thefirst stage occurs; but when the reaction is carried out above 0.,either in the presence or absence of a high-boiling solvent, e. g.,nitrobenzene, both stages proceed simultaneously. The ethyl chlorideformed in the second stage may be recovered quantitatively by chillingthe evolved gases. Pure ethyldichloroarsine may be obtained bydistilling through an efficient column the product obtained in thereaction described. The density of the distillate is 1.6570 for D53 Itboils at 825 C. under 75 mm. pressure.

As a specific example, th following directions convenient for thepreparation of ethyldichloroarsine in batches of about 5 lbs' are given.The reaction is carried out at above C. and in the absence of anysolvent. Both stages of the reaction proceed simultaneously.

Example 1 In a 5-liter, 3-neck flask equipped with a dropping funnelhaving a gas inlet side arm, an eficient mechanical stirrer and a refluxcondenser is placed 2,730 g. (15 moles) of arsenic trichloride. Air isswept out of the flask by passing dry nitrogen through the glass inlettube attached to the dropping funnel. This tube is then closed off withascrew clamp. The flask is placed in an oil bath heated to C.; and afterthe arsenic trichloride has reached approximately that temperature, afew cc. of tetraethyl lead is added from the dropping funnel to thestirred arsenic trichloride. The start of the reaction, which occurs ina few minutes, is indicated by clouding of the liquid and separation ofa white precipitate. A total of 1,620 g. (5 moles) of tetraethyl lead isthen added through the dropping funnel at such a rate that the reactionmixture is kept gently boiling. During this addition of the tetraethyllead and for one hour after the addition is complete, stirring iscontinued, and the oil bath is held at a temperature of about 100-110"C. About seven hours in all is required for these operations.

After the product has cooled to about room temperature, the flask isconnected with a Claisen still-head. The product is distilled directlyfrom the reaction mixture at 75 mm. pressure. The receiver, a 3-liter,round-bottom flask, is connected to a vacuum pump through a trap keptat. 80 C. During the distillation the oil bath is maintained at 120 C.;the product distills at 82-83 C. About 80% of the product distills overin four to live hours; the distillation of the re.-v mainder is slowerbecause of the large amount of lead chloride in the flask. The processmay be hastened by gradually reducing the pressure. About ten hours isrequired to distill over an amount equivalent to a 95% yield.

The density of the crude water-white product:

varied in four runs from 1.6735, to 1.6799 as com pared to 1.6570 forthe very pure material, dis.- tilled through a column. Hence thedistillate; obtained as described is 95.5 to 97% pure. The impurity isarsenic trichloride, the boiling point of which is 20% below that ofethyldichloroarsine. Fractionation thrOugh a column readily separatesthe two substances, and the arsenic trichloride may be recovered. Thepercentages thus obtained check well with the composition calculatedfrom the density of the crude material.

The described method with. slight modifica. tions' which are apparentcan. be applied on a commercial scale. On an industrial scale the crudereaction product may be separated from the precipitated lead chloride byfiltration. This typeof separation has been successfully carried out. Nodifficulties were experienced in carrying out the method described withproper precautions in handlingthe materials and in removing fumes Forthe purpose of comparison, arsenic trichloride was treated withtetraethyl lead in, a manner similar tothat described except in usingcarbon tetrachlorideasa solvent in the reaction mixture and treating themixture for about twelve hours at approximately the boiling point of thesolvent- The yield of, ethyldichloroarsine by this low-temperaturepreparation was v only 69% if the first stage reaction is used as abasis of, calculation, and is only 46% the. over-all reaction, is used.Similar results; were obtained when benzene (boiling point. of 78C.)or-ligroin were used as solvents and the reaction; carried out attemperatures below 80 C.

In another investigation the proportion of arsenie trichloride totetraethylleadwas increased to four moles to one. Benzene was used as asolvent, and the reaction was carried out. at below. 80? 0.. Therecovered arsenic trichloride amountedto 1.3' mole equivalent; theethyldichloroarsine obtained was equivalent to two moles of arsenictrichloride. These results. show thatat a temperature below 80C., alarge excess of" arsenic trichloride was capable of removing. only.twoethyl groups from the tetraethyl lead,

In all reactions conducted at. temperatures above 80 C., with or withoutsolvent,. where three moles of arsenic trichloride to one moleoftetraethyl' lead" were used, the yield; ofethyldichloroarsine approachedthetheoretical. For, ex-v ample, in a preparation in which no solventvvwas employed, the yield was 96.5%,.

The. liquid collected bycondensing: gas; evolved duringthe reaction, hada molecular weight COT.- responding to that of; ethyl chloride. andboiled at 12 C.v The solid residue was almost, pure lead chloride.

Anal; calcd. for PbClz: Cl, 25.5. 25.8.

When an equimolecular mixture of diethyllead dichloride and. arsenictrichloride was heated to 125 (3., there was a vigorous reaction andethyl chloridewas-evolved. The reaction mixture; was distilled at '75mm. The product obtained was ethyldichloroarsine in a yield of 75%..

Found: 01;

Reactions-similar to the one; used-'intheg- Prep-- aration ofethyldichloroarsine may be used for the preparation of a number ofrelated organomineralehalidea. in general, represented by the formulawherein Rn represents organic radicals, such as alkyl, aryl, alkaryl oraralkyl radicals; M represents a metal, non-metal or semi-metal, such asarsenic, phosphorus or antimony; and Xm stands forhalogen groups,vgenerally chloride or bromide groups; thesubscripts n and m being wholenumbers which. add up. to the valence of the constituent M, which isthree for the trivalent mineral' atom.

Preparations here described are those of (1) diethyl'cholorarsine(C2H5)2ASC1; (2) ethyldichlorophosphine C2H5PC12 and (3)ethyldichlorostibine. C2H5SbC12.

Example 2 lliethy lchloroorsine, preparations,..At. high te eratures,excess tetraethyl. lead. reacts with ethyldichloroarsine to form,diethyl'chloroarsine. Ethyldichloroarsine, 52.5 g. (Dd-mole) washeated.to (1., and 48.5 g. (0.15. mole). of, tetraethyl lead was slowly added.The separationof awhite precipitate indicated that reaction had,occurred; After the mixture had. been kept at. 120 C; for two hours, theproduct wasdistilled, directly from the reaction flask. A, yifel'dj Of39.9; g.. of diethylchloroarsine was obtained. This. product boiled at74-78 C. under '14 mmpressure.

Anal. calcd. for EtzAsCl: CI, 21.1. Found: CL, 0.-

The product is, therefore, mainly diethylchlop roarsine with a smallamount of triethylarsine.

The white, residue from. the reaction was, washed with benzene; itweighed 49 g. The the oretical yield was 50.5%,

Ana1.,calcd..f.or (C2Hsl2PbCl21 Cl, 212. Found: o1,.21.9.

Example-3;

Ethyldichlorophosphine preparation. Phosphorus trichloride, 69 g; (0.5mole)v was placed in a three-necked flask fitted with a; droppingfunnel, mechanical" stirrer and a reflux condenser. While a slow streamofv nitrogen was passed in-t' -thefiask, 54 g; (0.167 mole)of'tetraethyl lead wasadded: Reactionwas extremely slow; therewas noprecipitation of'leadiuntilthemixture had been refluxedfor two hours.The flask was heated in an oil bathat,11'0." C. until the mixtureceased; to reflux ('36, hours). The volatile material was then distilleddirectly from the reaction vessel. The 58.5 g. of colorless,evilsmelling distillate (B. P. 94-97=C. at 760mm.) represents ayield;of.8.9. per cent...

Anal. calcd. for CzHsPCl C1,. 54.2. Found: Cl, 56.2; 56.1:

Example, 4.

Ethyldichlorostibine preparaztz'on-r-elnv the ape paratus, for;preparing ethyldichloroarsine, 68.4 g. (0.3 mole) of dried andpulverized antimony trichloride was suspended in cc. of? solvent;Tetraethyl' lead (32.3 g;, 0.1: mole): was their addedslowly. Themixture washeat'ed underreflux-for eight hours and then cooled. Afterthesolvent had been removed by distillation, the residue was distilledunder: reduced pressure. The product,whichboiledbetween, 1 13? and 120C. at 25' mm. was redistilled. A total of 48.6 g. ofa colorless; liquidwas. obtainedz (5B1. 624 838 0. at 1 mm., D 2.182).

heat the halide under reflux and to control the rate of reaction by therate of addition of the organo-lead compound. The control is importantfor allowing the reaction to be brought up to the proper hightemperature level. It has been shown that desired products are notobtained in satisfactory yields unless the reaction is carried out at aproper temperature level.

At a sufficiently high reaction temperature level, which may vary withthe reactants and is in a number of instances of the order of 90 C. andhigher, the organo-lead compound, represented by PbR4 or RzPbXz, R.being a hydrocarbon radical and X a halide radical, tends to be reducedto PbX2, the dihalide. Thus, likewise, at a sufliciently elevatedreaction temperature, a PbR4 compound, such as tetraethyl lead, is madeto lose more than 2R (hydrocarbon) radicals from a molecule.

The organo-mineral-halides, and particularly theorgano-mineral-dihalides in which the mineral constituent is trivalent,have important uses as chemical warfare agents. In some instances theorgano groups may be methyl or phenyl radicals for greatereffectiveness. In the event the organo-metal halides lack the desiredviscosity or persistency, they may be used together with suitablethickening agents. For example, ethyldichloroarsine may be blended withcellulose acetatebutyrate. A 5% solution of this type is comparable inconsistency to glycerine and is quite stable. In addition to increasingthe viscosity of the agent, a thickening additive may aid in loweringthe volatility. The organo-mineral-halides may also be used in mixtureswith other toxic agents.

It is to be understood that modifications may bemade which come withinthe spirit and scope of the invention.

We claim:

1. A method of preparing an alkyldihaloarsine which comprises reacting atetra-alkyl lead compound, PbRi, in which R. represents an alkylradical, with an arsenic trihalide, AsXs, in which X 6 represents ahalide radical, at a temperature sufficiently elevated to remove morethan two alkyl radicals from molecules of the tetraalkyl lead and reducethe lead compound to a. lead dihalide.

2. A method of preparing ethyldichloroarsine which comprises reactingarsenic trichloride with tetraethyl lead by heating arsenic trichlorideunder reflux to a temperature above C. and adding tetraethyl lead to theheated arsenic trichlori-de at a controlled rate to control the reactionrate.

3. A method of preparing ethyldichloroarsine which comprises heatingarsenic trichloride to above C., then adding tetraethyl lead to the thusheated arsenic trichloride at a controlled rate to control the rate ofreaction.

4. A method of preparing ethyldichloroarsine Which comprises reactingarsenic trichloride with tetraethyl lead at a temperature of about toC., recovering evolved ethyl chloride, and separating lead dichloridefrom the reaction mixture.

5. A method of preparing ethyldichloroarsine which comprises reactingdiethyllead chloride with arsenic trichloride at a temperature above 90C.

6. A method of preparing diethylchloroarsine which comprises reactingethyldichloroarsine with tetraethyl lead at a temperature above 90 C.

MORRIS S. KHARASCH. SIDNEY WEINHOUSE.

References Cited in the file of this patent UNITED STATES PATENTS NameDate Kharasch et al Oct. 21, 1952 OTHER REFERENCES Number

1. A METHOD OF PREPARING AN ALKYLDIHALOARSINE WHICH COMPRISES REACTING ATETRA-ALKYL LEAD COMPOUND, PBR4, IN WHICH R REPRESENTS AN ALKYL RADICAL,WITH AN ARSENIC TRIHALIDE, ASX3, IN WHICH X REPRESENTS A HALIDE RADICAL,AT A TEMPERATURE SUFFLCIENTLY ELEVATED TO REMOVE MORE THAN TWO ALKYLRADICALS FROM MOLECULES OF THE TETRAALKYL LEAD AND REDUCE THE LEADCOMPOUND TO A LEAD DIHALIDE.